1. Field of the Invention
The present invention relates to a dragline for digging and transporting rock and stones, soil and sand, minerals, etc. (as will be hereinafter called merely rocks and stones), and particularly to a dragline having built-in hopper and loading means for hauling rocks and stones.
2. Prior Art
In recent years there has been a need for high-production and high efficiency excavation methods in large scale land creating works, foreshore reclamation works, surface coal mining works on coal seams having thick overburden, or the like. One conventional method of digging soil and rock in the open air was to effect excavation by a shovel loader and carrying away the excavated material by trucks. Another method was to effect excavation by a bucket wheel excavator and haul the excavated material on a belt conveyor. Any of these prior art methods has been unsatisfactory with respect to the high capacity and efficiency when they were applied to large-scale works. Both the shovel loader and the bucket wheel excavator were inherently limited in size of their shovel or bucket because of their structure, hence they had a limited capacity. In addition they were particularly unsuitable for handling large masses of soil and rock, so that many manhours were required for blasting operations to prevent such large masses from being produced or for boulder blasting operations when great lumps were produced. The aforesaid former method is undesirable especially in excavating places involving bumpy roads because trucks are subject to severe damages.
Digging by the use of a high capacity dragline is known in a vast amount of rock removing work on overlying strata (overburden) above a coal seam as in a strip coal mining. This method provides a very high efficiency and large capacity operation in that the overlying rock as dug by the dragline are dumped and piled directly on the gob or waste area without using any intermediate transport means. However, in mining multiple coal strata having more than two coal seams it may be impossible to carry out the mining operation on the second and lower coal seams when the total thickness of the overlying rock layers exceeds the capacity of a dragline. Practically, therefore, it has been heretofore a usual practice to mine only the first uppermost coal seam even in the case of a multiple coal strata, or at most to dig out the upper layer of rock overlying the first coal seam by a power shovel, haul the excavated material to another place by trucks and then use draglines to dig the overburden of the second coal seam.
Even in the case of a single coal stratum, if there is a large thickness of overburden, there will be a correspondingly increased quantity of waste produced, so that an increased proportion of the waste which has once been dumped at one place must be again transferred to another distant place, resulting in decreasing the efficiency in operation. Furthermore, the boom of a dragline must be swung through an arc of more than 90.degree. up to approximately 180.degree. in operation in order to dump the waste as far as possible, resulting in extending the time required per cycle of the bucket, hence a decreased efficiency. This is due to the dragline's characteristics that despite its great digging capacity the transporting distance is limited to the length of its boom or at most the order of 100 m.
Belt conveyors are known as a large capacity and high efficiency haulage means. Shiftable conveyors capable of lateral movements are particularly suitable for use at a mining area where mobility of the transport means is required. Mining operation is composed primarily of digging and transporting operations. In the past, however, there has been no mining process employed involving a combination of a dragline as described which is a large capacity and high efficiency excavator and a belt conveyor which is likewise a high capacity and high efficiency transporter. One of the reasons is attributed to the dragline's characteristics. That is, since the dragline swings its boom to move its vast bucket filled with the excavated soil and rock (as will hereinafter be referred to as excavated or dug material), the dragline is best suited to dump the excavated material while scattering it over some extent of area. But it has difficulties in dumping the excavated material onto a particular small target such as a hopper or the deck of a truck. If this is to be done, it would take much time to position the dragline itself such that the bucket may be brought to a position directly above the hopper. It would also take a lot of time to bring the bucket to a halt just above the hopper in each cyclic operation between scraping and dumping actions by the bucket, resulting in an extended cycle time of the bucket and a decrease in efficiency. The dragline could not thus exhibit its inherent special performance. Conversely, if the bucket were allowed to dump the material over a considerable extent of region, the hopper should be an enormous one in size enough to receive the moving bucket. Even though it were made possible to hold the bucket size down to a certain extent by spending much time in controlling the movement of the bucket as described above, the bucket would still be of a considerable size and should be capable of movement as the dragline is moved around. Such movable hoppers have not heretofore been proposed.
Another reason that the combination of the dragline and belt conveyor has not been used lies in the belt conveyor. Materials dug often contain big masses of rock or stone. While the dragline can scrape up such big lumps by its vast bucket, ordinary hoppers or belt conveyors cannot accommodate or handle big lumps. For the foregoing reasons any mining system utilizing a combination of draglines and belt conveyors has not been conceived of in the past.